1. Field of the Invention
The present invention generally relates to a display device and more particularly, to a matrix-type display device.
2. Description of the Prior Art
Generally, the matrix-type display device referred to above has an advantageous feature in that it can be formed flat because it has a display unit provided at an intersection between a line electrode bus and a row electrode bus. However, in this matrix-type display device, an interruption or a breaking of a bus would cause trouble in that a plurality of display elements connected to the interrupted or broken bus are rendered inoperative thereby giving rise to an erroneous disconnection of the display lines. The erroneous disconnection of the display lines is a fatal damage to the display device, and accordingly, a display device having a broken bus, even a single one, must be discarded as a defective device. In view of this, the following prior art techniques have been already proposed in order to reduce the bad influence of the breaking of a bus as stated above.
(1) A bus transmitting the same signal is provided in duplicate.
This method is based on the fact that a defect can be backed up and compensated for with the duplicated function if the necessary functions are duplicated. In general, spare lines are provided for countermeasures against the breaking of the bus lines. The Japanese Laid-Open Patent Publications (unexamined) Tokkaisho Nos. 56-90497, 56-153588, 56-153589, etc. disclose a method for providing the bus in duplicate.
(2) A first bus material is layered with a second bus material.
For example, in a liquid crystal display device driven by a thin film transistor (TFT), if the bus material transmitting a scanning signal is layered with the bus material transmitting a display signal, electricity can be transmitted through the layer of another bus material even in the case where one layer is defective.
(3) A bus is overlapped with a driving part which transmits a signal to the bus.
This method disclosed in the Japanese Laid-Open Patent Publication (unexamined) Tokkaisho No. 56-153587 is considered effective particularly when the display part is integrally combined with the driving part. However, in the case that the driving part and the display part and manufactured separately and then connected to each other, the number of mounting operations is undesirably increased.
Since the breaking of the bus raises a very serious problem on the matrix-type display device which has a large number of display picture elements, various kinds of methods have been contrived heretofore so as to solve the problem. According to the above-described first prior art (1), spare lines which are originally not necessary for the display device are provided. Therefore, it is disadvantageous for a display device of a transparent type in that the area which light passes through, that is, the opening ratio is inevitably reduced to render the display dark. The second prior art (2) relates to how the breaking of the line can be prevented from taking place. To carry out the method according to this prior art, it is necessary to open a window for connections of the bus materials, therefore requiring that, with consideration as to the minimum width necessary for opening the window, the bus itself must be larger than that needed in the case of a display device without a contact window. As a result of this, the opening ratio is reduced. Moreover, it is impossible from the viewpoint of the principle that both materials are layered one by one in the thin film transistor. Therefore, the second method has no effect with respect to the breaking of a line in the thin film transistor. The third method is useful particularly when a special material is employed so that the display part can be integrally layered with the driving part. In other words, when the necessary frequency for treating the related circuits is taken into consideration, the material to be employed in the third method is restricted to one which has a large electron mobility. Although it is possible to form a thin film transistor of the display part with amorphous silicon or polysilicon, etc. which is generally used as a material for the thin film transistor, it is impossible at present to form the driving part which can perform signal treating. Thus, the third method cannot be applied when a material such as amorphous silicon or polysilicon, etc. is used, and it is inconvenient should the driving part and the display part be manufactured separately and individually so as to be connected to each other later. In this case, the number of connections, namely, the number of mountings is considerably increased.